mirror of
https://github.com/RPCS3/llvm.git
synced 2024-12-03 17:32:59 +00:00
8eef4b2d05
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43247 91177308-0d34-0410-b5e6-96231b3b80d8
264 lines
9.0 KiB
HTML
264 lines
9.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
|
|
<html>
|
|
<head>
|
|
<title>Kaleidoscope: The basic language, with its lexer</title>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<meta name="author" content="Chris Lattner">
|
|
<link rel="stylesheet" href="../llvm.css" type="text/css">
|
|
</head>
|
|
|
|
<body>
|
|
|
|
<div class="doc_title">Kaleidoscope: The basic language, with its lexer</div>
|
|
|
|
<div class="doc_author">
|
|
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="intro">Tutorial Introduction</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
|
|
will run through implementation of a simple language, showing how fun and easy
|
|
it can be. This tutorial will get you up and started and build a framework you
|
|
can extend to other languages and to play with other things.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="language">The basic language</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This tutorial will be illustrated with a toy language that we'll call
|
|
"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>".
|
|
Kaleidoscope is a procedural language that allows you to define functions, use
|
|
conditionals, math, etc. Over the course of the tutorial, we'll extend
|
|
Kaleidoscope to support if/then/else, operator overloading, JIT compilation with
|
|
a simple command line interface, etc.</p>
|
|
|
|
<p>Because we want to keep things simple, in Kaleidoscope the only datatype is a
|
|
64-bit floating point type (aka 'double' in C parlance). As such, all values
|
|
are implicitly double precision and the language doesn't require type
|
|
declarations. This gives the language a very nice and simple syntax. For
|
|
example, A simple example computes <a
|
|
href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers</a>,
|
|
which looks like this:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
# Compute the x'th fibonacci number.
|
|
def fib(x)
|
|
if x < 3 then
|
|
1
|
|
else
|
|
fib(x-1)+fib(x-2)
|
|
|
|
# This expression will compute the 40th number.
|
|
fib(40)
|
|
</pre>
|
|
</div>
|
|
|
|
<p>We also allow Kaleidoscope to call into standard library functions (this LLVM
|
|
JIT makes this completely trivial). This means that you can use the 'extern'
|
|
keyword to define a function before you use it (this is also useful for mutually
|
|
recursive functions). For example:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
extern sin(arg);
|
|
extern cos(arg);
|
|
extern atan2(arg1 arg2);
|
|
|
|
atan2(sin(.4), cos(42))
|
|
</pre>
|
|
</div>
|
|
|
|
<p>In the first incarnation of the language, we will only support basic
|
|
arithmetic: if/then/else will be added in a future installment. Another
|
|
interesting aspect of the first implementation is that it is a completely
|
|
functional language, which does not allow you to have side-effects etc. We will
|
|
eventually add side effects for those who prefer them.</p>
|
|
|
|
<p>In order to make this tutorial
|
|
maximally understandable and hackable, we choose to implement everything in C++
|
|
instead of using lexer and parser generators. LLVM obviously works just fine
|
|
with these tools, and choice of these tools doesn't impact overall design.</p>
|
|
|
|
<p>A note about this tutorial: we expect you to extend the language and play
|
|
with it on your own. Take the code and go crazy hacking away at it. It can be
|
|
a lot of fun to play with languages!</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="language">The Lexer</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>When it comes to implementing a language, the first thing needed is
|
|
the ability to process a text file and recognize what it says. The traditional
|
|
way to do this is to use a "<a
|
|
href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
|
|
to break the input up into "tokens". Each token returned by the lexer includes
|
|
a token code and potentially some metadata (e.g. the numeric value of a number).
|
|
First, we define the possibilities:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
|
|
// of these for known things.
|
|
enum Token {
|
|
tok_eof = -1,
|
|
|
|
// commands
|
|
tok_def = -2, tok_extern = -3,
|
|
|
|
// primary
|
|
tok_identifier = -4, tok_number = -5,
|
|
};
|
|
|
|
static std::string IdentifierStr; // Filled in if tok_identifier
|
|
static double NumVal; // Filled in if tok_number
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Each token returned by our lexer will either be one of the Token enum values
|
|
or it will be an 'unknown' character like '+' which is returned as its ascii
|
|
value. If the current token is an identifier, the <tt>IdentifierStr</tt>
|
|
global variable holds the name of the identifier. If the current token is a
|
|
numeric literal (like 1.0), <tt>NumVal</tt> holds its value. Note that we use
|
|
global variables for simplicity, this is not the best choice for a real language
|
|
implementation :).
|
|
</p>
|
|
|
|
<p>The actual implementation of the lexer is a single function <tt>gettok</tt>.
|
|
<tt>gettok</tt> is called to return the next token from standard input. Its
|
|
definition starts as:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
/// gettok - Return the next token from standard input.
|
|
static int gettok() {
|
|
static int LastChar = ' ';
|
|
|
|
// Skip any whitespace.
|
|
while (isspace(LastChar))
|
|
LastChar = getchar();
|
|
</pre>
|
|
</div>
|
|
|
|
<p>
|
|
<tt>gettok</tt> works by calling the C <tt>getchar()</tt> function to read
|
|
characters one at a time from standard input. It eats them as it recognizes
|
|
them and stores the last character read but not processed in LastChar. The
|
|
first thing that it has to do is ignore whitespace between tokens. This is
|
|
accomplished with the loop above.</p>
|
|
|
|
<p>The next thing it needs to do is recognize identifiers, and specific keywords
|
|
like "def". Kaleidoscope does this with this simple loop:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
|
|
IdentifierStr = LastChar;
|
|
while (isalnum((LastChar = getchar())))
|
|
IdentifierStr += LastChar;
|
|
|
|
if (IdentifierStr == "def") return tok_def;
|
|
if (IdentifierStr == "extern") return tok_extern;
|
|
return tok_identifier;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>Note that it sets the '<tt>IdentifierStr</tt>' global whenever it lexes an
|
|
identifier. Also, since language keywords are matched by the same loop, we
|
|
handle them here inline. Numeric values are similar:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
|
|
std::string NumStr;
|
|
do {
|
|
NumStr += LastChar;
|
|
LastChar = getchar();
|
|
} while (isdigit(LastChar) || LastChar == '.');
|
|
|
|
NumVal = strtod(NumStr.c_str(), 0);
|
|
return tok_number;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>This is all pretty straight-forward code for processing input. When reading
|
|
a numeric value from input, we use the C <tt>strtod</tt> function to convert it
|
|
to a numeric value that we store in <tt>NumVal</tt>. Note that this isn't doing
|
|
sufficient error checking: it will incorrect read "1.23.45.67" and handle it as
|
|
if you typed in "1.23". Feel free to extend it :). Next we handle comments:
|
|
</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
if (LastChar == '#') {
|
|
// Comment until end of line.
|
|
do LastChar = getchar();
|
|
while (LastChar != EOF && LastChar != '\n' & LastChar != '\r');
|
|
|
|
if (LastChar != EOF)
|
|
return gettok();
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>We handle comments by skipping to the end of the line and then returnning the
|
|
next comment. Finally, if the input doesn't match one of the above cases, it is
|
|
either an operator character like '+', the end of file. These are handled with
|
|
this code:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
// Check for end of file. Don't eat the EOF.
|
|
if (LastChar == EOF)
|
|
return tok_eof;
|
|
|
|
// Otherwise, just return the character as its ascii value.
|
|
int ThisChar = LastChar;
|
|
LastChar = getchar();
|
|
return ThisChar;
|
|
}
|
|
</pre>
|
|
</div>
|
|
|
|
<p>With this, we have the complete lexer for the basic Kaleidoscope language.
|
|
Next we'll <a href="LangImpl2.html">build a simple parser that uses this to
|
|
build an Abstract Syntax Tree</a>. When we have that, we'll include a driver
|
|
so that you can use the lexer and parser together.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
|
|
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
|
|
</address>
|
|
</body>
|
|
</html>
|